Generic menu items and lambda discount

import java.util.*;

public class Main {
    private static final Scanner IN = new Scanner(System.in).useLocale(Locale.ROOT);

java.util.* imports utility type names such as Scanner, Locale and collections. Main is the public class matching Main.java. The private static final IN field holds one shared Scanner for System.in; final prevents replacing that reference. The scanner uses Locale.ROOT.

    static String text() {
        if (!IN.hasNextLine()) throw new IllegalArgumentException("Incomplete input.");
        String value = IN.nextLine().trim();
        if (value.isEmpty() || value.length() > 200) throw new IllegalArgumentException("Text must have 1–200 characters.");
        return value;
    }

text checks whether a line exists, reads it and removes leading/trailing whitespace. Reject empty text and lengths above 200 by throwing IllegalArgumentException. Otherwise return the string. One field is read per line, so these helpers do not mix nextInt with leftover newlines.

    static int integer(int low, int high) {
        int value = Integer.parseInt(text());
        if (value < low || value > high) throw new IllegalArgumentException("Integer out of range.");
        return value;
    }

Integer.parseInt converts the returned string to int or throws NumberFormatException, a subtype of IllegalArgumentException. Reject values outside the caller’s inclusive low/high limits, then return the accepted integer.

    static double number(double low, double high) {
        double value = Double.parseDouble(text());
        if (!Double.isFinite(value) || value < low || value > high) throw new IllegalArgumentException("Number out of range.");
        return value;
    }

Double.parseDouble reads the string as a double. Double.isFinite rejects NaN and infinities as well as the explicit low/high range check. Return only a finite accepted value; exceptions transfer control to a matching catch block.

    static void money(String label, double value) { System.out.printf(Locale.ROOT, "%s: %.2f%n", label, value); }

money is a formatting helper: %s prints the label, %.2f prints two decimal places, and %n ends the line. Locale.ROOT fixes locale-sensitive formatting. The helper can also display non-currency quantities; it does not change the stored value.

static class MenuItem<T1,T2> {
        final T1 itemName;final double price;final T2 category;
        MenuItem(T1 name,double price,T2 category){if(!Double.isFinite(price)||price<0||price>1000000)throw new IllegalArgumentException("Invalid price.");itemName=name;this.price=price;this.category=category;}
    }

MenuItem can store different reference types for its name and category. Price remains double and is validated in the constructor. The final fields are assigned for each created item.

    static double generateBill(List<? extends MenuItem<?,?>> items){double total=0;for(MenuItem<?,?> item:items)total+=item.price;return total;}

The extends wildcard accepts compatible lists of MenuItem instances. The method only reads each item and adds its price; it does not need to know the concrete name/category type arguments.

    public static void main(String[] args){
        try{
            int n=integer(1,50);ArrayList<MenuItem<String,String>> items=new ArrayList<>();
            for(int i=0;i<n;i++)items.add(new MenuItem<>(text(),number(0,1000000),text()));

main uses ArrayList>. Each loop creates an item from name, finite price and category lines. The diamond syntax lets Java infer the constructor type arguments.

            double total=generateBill(items);java.util.function.DoubleUnaryOperator discount=value->value>1500?value*0.90:value;

generateBill computes the subtotal. DoubleUnaryOperator maps one double to another: totals strictly above 1500 receive 10% off; exactly 1500 does not satisfy the > condition.

            for(MenuItem<String,String> item:items)System.out.println(item.itemName+" | "+item.category);
            money("Subtotal",total);money("Discount",total-discount.applyAsDouble(total));money("Bill",discount.applyAsDouble(total));

Print each item’s name/category. The discount amount is subtotal minus the lambda’s discounted result, and Bill is that result. Applying this stateless lambda twice gives the same answer.

        }catch(IllegalArgumentException e){System.out.println("Invalid input: "+e.getMessage());}
    }
}

The catch handles IllegalArgumentException, including invalid numeric strings from the helpers, and prints its message. The final braces close main and Main. Java objects become eligible for garbage collection when no live reference needs them; this program does not manually free them.